Hydraulic-pneumatic weight control and compensating apparatus

ABSTRACT

Apparatus automatically compensating for relative vertical movement between a hoisting or supporting mechanism and a load carried thereby, which load, for example, may be a running string connected to a drill bit used in drilling a sub-aqueous well bore, the mechanism supporting the running string being mounted on a floating vessel anchored over the well bore. The compensating apparatus includes a cylinder and piston device containing a hydraulic fluid exerting a lifting or tensioning force on the running string, or other load, the pressure on the hydraulic fluid being maintained generally constant by a gaseous medium despite relative axial movement between the cylinder and piston portions of the device, that might result from heaving of the vessel due to wind and wave action, or the lowering of the running or drilling string as the bit drills the hole.

United States Patent Larralde et al.

[ 1 Feb. 27, 1973 1 HYDRAULIC-PNEUMATIC WEIGHT CONTROL AND COMPENSATING APPARATUS [75] Inventors: Edward Larralde, Santa Barbara;

Ronald E. Beaufort, Downey, both of Calif.

[73] Assignee: Vetco Offshore Industries, Inc.,

Ventura, Calif.

[22] Filed: Sept. 4, 1970 21 Appl. No.: 69,758

[52] US. Cl. ......254/172, 254/173, 175/5 [51] Int. Cl. ..B66d 1/48 [58] Field of Search ..254/l72, 173; 166/.5; 175/5;

[56] References Cited UNITED STATES PATENTS 3,208,728 9/1965 Parks 254/172 3,259,371 7/1966 Goepfert et al. 254/173 3,343,810 9/1967 Parnell ..254/172 3,403,728 10/1968 Richardson et al..... 254/172 X 3,158,206 11/1964 Kammerer ..l75/5 3,158,208 11/1964 Kammerer... 175/5 X 3,421,581 l/l969 Van Geijn 17515 X 3,396,741 8/1968 Wintterlin et al. ..l37/ll5 3,524,465 8/1970 Sadler 1 37/1 15 FOREIGN PATENTS OR APPLICATIONS 315,176 2/1934 ltaly ..254/l72 Primary Examiner-Evon C. Blunk Assistant ExaminerMerle F. Maffei Attorney-Bernard Kriegel [5 7] ABSTRACT Apparatus automatically compensating for relative vertical movement between a hoisting or supporting mechanism and a load carried thereby, which load, for example, may be a running string connected to a drill bit used in drilling a sub-aqueous well bore, the mechanism supporting the running string being mounted on a floating vessel anchored over the well bore. The compensating apparatus includes a cylinder and piston device containing a hydraulic fluid exerting a lifting or tensioning force on the running string, or other load, the pressure on the hydraulic fluid being maintained generally constant by a gaseous medium despite relative axial movement between the cylinder and piston portions of the device, that might result from heaving of the vessel due to wind and wave action, or the lowering of the running or drilling string as the bit drills the hole.

4 Claims, 4 Drawing Figures HYDRAULIC-PNEUMATIC WEIGHT CONTROL AND COMPENSATING APPARATUS The present invention relates to apparatus for controlling the stress in a running string, and more particularly to apparatus used on or in connection with a floating vessel for maintaining the strain in a running string (such as a pipe string or cable) substantially constant while being used in the performance of diverse functions in a sub-aqueous well bore, such asdrilling and completion operations therein, despite vertical movement of the vessel while such operations are being performed.

In the normal operation of drilling a well bore on land, or from a drilling platform supported in a fixed position from the ocean floor, the weight on the drill bit is equal to the total weight of the drilling string less the weight of the drill pipe carried by the drawworks. Usually, the weight imposed on the bit is equal to the weight of the drill collar sections connected to the lower end of the drill pipe. ln drilling a sub-aqueous well bore from a floating vessel, problems of compensating for the weight on the bit arise due to the heaving of the vessel under conditions of tide, wind and waves. At the present time, such compensation is generally accomplished through use of a slidable spline connection in the drill string above the drill collars. Although such slidable spline connection is widely used, it presents many difficulties, the principal one being the requirement that it transmit torque. When subjected to high torque, sufficient friction is developed in the splines as to render the free sliding of the joint ineffective. Moreover, such slidable spline connections have comparatively low torsional strength, resulting in their failure, with attendant very high fishing costs. At times, the result of the failure has been the abandonment of the well.

Motion compensating devices have been proposed for overcoming the aforenoted difficulty, in which the drilling string is supported hydraulically by interposing a compensating apparatus between the travelling block and hook of the usual drilling apparatus employed in drilling the well bore. Such types of apparatus, and similar apparatus, are illustrated in US. Pat. Nos. 2,945,676, 2,945,677, 3,151,686, 3,158,206 and 3,158,208. In general, the devices illustrated therein rely upon the maintenance of a predetermined fluid pressure in a cylinder and piston mechanism by circulating hydraulic fluid under the required pressure continuously through the cylinder. The necessity for continuously pumping the hydraulic fluid at high pressure through the compensator requires a pump driven by a motor of large horsepower. So far as known, the systems illustrated in the above patents have not been used.

By virtue of the present invention, a motion compensating apparatus is provided in which it is unnecessary to constantly circulate hydraulic fluid through the apparatus. Instead, a gaseous medium under pressure maintains the required pressure in the hydraulic fluid in the cylinder and piston device in the absence or presence of longitudinal movement of the cylinder and piston portions of the device with respect to each other, the hydraulic fluid exerting a continuous and substantially constant stress on a running string, such as a drilling string or cable. When the apparatus is used in the drilling of a sub-aqueous well bore from a floating vessel, the stress exerted is a substantially constant lifting force on the drilling string despite heaving of the drilling vessel in the water, thereby insuring the maintenance of the desired drilling weight on the drill bit secured to the lower end of the drilling string. The pressure of the gaseous medium can be varied to vary the lifting force on the drilling string and thereby determine the drilling weight on the drill bit. In its more general aspects, the apparatus is useful generally in controlling the tension in a lifting mechanism, such as in a hoist, despite the tendency for the load on the hoist to be partially or totally relieved in a sudden fashion, as, for example, as a result of lowering a load onto a floating and heaving barge, to prevent the load from coming to rest upon the deck of the barge under impact conditions.

An object of the present invention is to provide an improved weight control and compensating apparatus used on a floating vessel that permits the vessel to move vertically with respect to suspended pipe, or other equipment, while maintaining the desired tension on the pipe or such equipment. If desired, such tension or strain may be varied to accommodate changes in equipment weight or weight of a suspended pipe string, such as a drilling string, or other tubular string, extending into a sub-aqueous well bore.

Another object of the invention is to provide a hydraulic-pneumatic weight control and compensating apparatus, particularly useful in drilling a sub-aqueous borehole from a floating drilling vessel, and comprising telescopically arranged cylinder and piston mechanisms, in which the relative position between the cylinder and piston can be indicated remotely to the operator, all in a relatively simple manner.

A further object of the invention is to provide an apparatus including a piston mechanism reciprocable in a companion cylinder, the stress in a running string or line being supported through a liquid medium confined in the cylinder and acting between the piston and the cylinder head, the desired pressure on the liquid medium being imposed thereon through use of an accumulator containing a gas at a desired pressure acting on the liquid medium, the gas expanding and contracting as the piston moves relatively longitudinally in the cylinder so as to maintain generally aconstant pressure on the liquid. Shifting in the relative position of the piston in the cylinder can occur readily, inasmuch as the relative motion is permitted and compensated for by expansion or contraction of the gas in the accumulalot.

A further object of the invention is to provide a hydraulic-pneumatic weight control and compensating apparatus, in which automatic means are provided for adding liquid to the system if required; conversely, to bleed excess liquid from the system that is not required.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from a consideration of several forms in which it may be embodied. Such forms are shown in the drawings accompanying and forming part of the present specification. These forms will now be described in detail for the purpose of illustrating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense.

Referring to the drawings:

FIG. 1 is a diagrammatic view of a drilling rig mounted on a floating vessel for drilling a sub-aqueous bore hole;

FIG. 2 is a front elevational view, with parts in longitudinal section, of the compensator portion of the apparatus disclosed in FIG. 1;

FIG. 3 is a diagrammatic view of the compensator system;

FIG. 4 is a side elevational view of another embodiment of the invention.

Apparatus is disclosed in FIGS. 1 to 3 in connection with the drilling of a vertical well bore W from a subaqueous floor F above which a floating vessel V, such as a drilling barge, is located, the barge being suitably anchored against lateral displacement for the purpose of holding a tubular drilling string S in centered relation with respect to the well bore. A drill bit B is secured to the lower end of the drill string, such as a string of drill pipe or drill casing, the upper kelly portion K of the drill string passing through the usual rotary table T rotated by a suitable drive mechanism (not shown). The upper end of the kelly is secured to a swivel R suspended from a hook C pivotally connected to the lower end of a compensating apparatus D, the upper end of which is pivotally connected to the travelling block E associated with the usual lines I-l passing over the crown block J at the upper end of the derrick L secured in place on the floating drilling vessel, the cables or lines being connected to a suitable drawworks N mounted on the floating barge.

A suitable mud line P is connected to the swivel for the purpose of delivering drilling mud to the drill string for discharge from the drill bit B, to remove the cuttings produced by the latter while appropriate drilling weight is imposed thereon, with the drilling string being rotated by the table T at the desired speed. Usually, the drilling weight is provided by a suitable length of drill collars disposed in the lower portion of the drill string immediately above the drill bit, the drill string above the drill collars being maintained in tension by the drawworks N and lines H, the drawworks permitting the drill string to lower as the hole W is being produced.

The compensating apparatus D permits the floating vessel V, and the mechanism carried thereby, to shift vertically relative to the well bore W and drilling string S without appreciably modifying the drilling weight imposed on the drill bit 13. This compensating apparatus includes a lower supporting structure connected to the hook C, and an upper supporting structure 11 connected to the lower end of the travelling block E. In the specific form of compensating apparatus illustrated, the lower supporting structure 10 is secured to the lower ends of a pair of piston rods 12 extending upwardly into a pair of cylinders 13 affixed to the upper supporting structure 11. The upper ends of the piston rods are secured to pistons 14 having appropriate seal rings (not shown) thereon for slidably sealing against the cylinder walls, the rod ends of the cylinders carrying appropriate packings 12a for slidably sealing against the periphery of the piston rods 12. The cylinders 13 are disposed on opposite sides of the travelling block E and extend thereabove, to reduce the overall length of the compensating unit D while permitting the pistons 14 and rods 12 to shift longitudinally within the cylinders to a substantial extent, the pistons, when disposed at the upper, head ends of the cylinders, still permitting the drawworks to elevate the travelling block and compensating unit to an extent at which the usual length of drill pipe can be connected to, or removed from, the drilling string. The arrangement of the compensating apparatus D with respect to the travelling block E and hook C forms the subject matter of the application of Edward Larralde and James W. E. I-Ianes, for Motion Compensating Apparatus, Ser. No. 69,759, Filed Sept. 4, 1970.

It is evident that the travelling block E is secured to the cylinders 13 so that the latter move vertically therewith, while the pistons 14 and piston rods 12 are secured to the hook C, and through the swivel R to the upper end of the drill string S. The weight of the drill string is transmitted through the hook C to the piston rods 12 and pistons 14, and then to liquid 15 filling the cylinder spaces below the pistons 14, from where it is transmitted to the lower cylinder heads 13a and to the cylinders 13 themselves, from where the load is transferred to the travelling block E and the lines H to the crown block J. As noted above, elevation and descent of the travelling block, and, therefore, of the compensating unit D and the entire load S suspended therefrom, is determined by the operation of the drawworks N.

A substantially constant predetermined pressure is maintained on the liquid medium 15 disposed in the cylinders. Such liquid under pressure acts in an upward direction on the pistons 14 and, therefore, on the hook.

C, swivel R and drill string S connected thereto. Since the weight of the entire drill string is known, the unit pressure of the liquid medium is selected such that all of such weight, with the exception of the drilling weight to be imposed on the drill bit B, which is usually the weight of the drill collars, is supported by the liquid pressure acting in an upward direction over the areas of the pistons. This pressure is derived from a gaseous medium 16, such as nitrogen, disposed in a bank of accumulators 17 supported on the vessel V. The rod ends of the cylinders 13 are connected to liquid lines 18 extending to a control valve 19, from where fluid lines 20 run to the liquid ends 21 of the cylindrical accumulator members 22. The lower portions of the accumulators are filled with the liquid 15, and the upper ends 23 are filled with the gas 16 under pressure, the gas and liquid in each cylindrical accumulator being separated by a floating piston 24 that makes a suitable slidable seal against the cylindrical wall of the accumulator housing 22. Accordingly, the gas pressure of the gas 16 is transmitted through the floating piston 24 to the liquid 15, the same liquid pressure being present in the compensator cylinders 13. Movement of the pistons 14 in the cylinders 13 is permitted, the floating pistons 24 correspondingly shifting while transmitting the pressure of the gaseous medium 16 to the liquid 15 in the accumulators, and thence through the lines 20, 18 to the liquid 15 in the compensator cylinders 13.

The ability of the liquid to pass between the compensator cylinders and the accumulators is controlled by the valve 19 of any suitable type. It may be of the air actuated type, in which a spring 25 normally biases the valve to an open condition. The valve may be closed by the operator manipulating a manually operated control valve 26, to permit air under pressure from a suitable source (not shown) to pass to a valve inlet 27 and then into the valve body for action on a diaphragm or piston (not shown) to shift the valve 19 to a closed position against the force of the spring 25. When the valve 19 is to be permitted to reopen, the manual valve 26 is manipulated to shut off the air supply from the inlet line 27 and permit the air in the diaphragm or piston chamber (not shown) to pass to atmosphere through the exhaust line 28. Normally, during the operation of the compensator apparatus, the valve 19 is in its open position to permit the free transfer of liquid between the compensator cylinders 13 and the accumulator cylinders 22.

A high pressure gas supply is contained in a bank of storage tanks 29 communicating with a manifold 30, there being a shut-off valve 31 between each storage tank and the manifold. The manifold line has a valve 32 therein normally biased to a closed position by a spring 33, but shiftable manually to an open position whenever the high pressure gas is to be permitted to flow through a variable pressure regulator valve 34, discharging therefrom at the selected pressure into lines 35, 36, leading to the accumulator cylinders 22. Each line 36 leading to its associated cylinder is controlled by a valve 37 normally biased to a closed position by a spring 38, but manually shiftable to an open position to permit the gas at the regulated pressure to pass into the accumulator gas chamber 23. A suitable gauge 37a in each line leading to an accumulator cylinder indicates the pressure therein, the gas pressures in the accumulator cylinder portions 23 all being equal.

While only a few accumulators l7 and storage vessels 29 are disclosed, for purpose of illustration only, actually they can be supplied in any number, all connected in the system as those illustrated.

The liquid medium is derived from a reservoir 38a, a suction line 39 from the reservoir running to a pump 40, the discharge line 41 of which can force the liquid through a check valve 42, and through a normally closed solenoid valve 43 into the liquid line communicating with the liquid containing portions 21 of the accumulator cylinders. The check valve 42 permits flow of liquid from the pump in a direction toward the accumulator cylinders 22 and the compensator cylinders 13, but will prevent reverse flow of such liquid to the pump.

The solenoid operated valve 43 is normally biased by a spring 44 to a closed position, being opened only upon the gas pressure in the accumulator l7 rising to a predetermined value above the hydraulic fluid pressure in the compensator cylinders 13 and the accumulators 17. When this occurs, the pump 40 delivers additional liquid from the reservoir 38a to the hydraulic or liquid portion of the system. The pump is driven by an air motor 45 in which air under pressure is constantly being supplied, although the supply of air can be shutoff by means ofa suitable valve 46 in the air supply line 47, if desired. The pump is of the stall type, and if the solenoid valve 43 is closed, the air pressure cannot operate the motor 45 to deliver liquid. However, upon opening of the solenoid valve 43, the motor immediately begins actuating the pump 40 to supply additional liquid to the accumulator cylinders 22 and the compensator cylinders 13.

As illustrated in the drawings, a differential pressure switch 48 controls the circuit to the solenoid 49 of the valve 43. Thus, power from a suitable source has one line 50 running to the solenoid coil 49 and another line 51 running to a contact 52 of the switch. The other contact 53 of the switch is connected to a line 54 connected to the solenoid coil. A switch arm 55 may bridge the contacts 52, 53 whenever the circuit to the solenoid is to be completed, this switch arm being connected to the free ends of a pair of bourdon tubes 56, 57 forming the pressure actuated elements of the differential pressure switch. One of these bourdon tubes 56 is connected, through a line 36a, to the gas lines 36 connected to the gas cylinder portions 23 of the accumulators 17; whereas, the other bourdon tube 57 is connected to a liquid line 58 communicating with the liquid line 20 communicating with the liquid portions 21 of the accumulators and the hydraulic cylinders 13.

Normally, springs 58a bias a bridging contact 59 away from the contacts 52, 53, or to an open condition, the switch remaining open so long as the pressure in the bourdon tubes 56, 57 are substantially equal. However, in the event that the pressure in the gas bourdon tube 56 increases significantly with respect to the pressure in the hydraulic bourdon tube 57, or, if the latter pressure decreases significantly with respect to the gas pressure,

7 then the gas tube 56 will expand, or the hydraulic bourdon tube 57 will contract, as the case may be, causing the switch arm 55 to close the switch and complete the circuit to the solenoid 49, the latter shifting the valve 43 to an open condition, whereupon the air operated motor 45 immediately drives the pump 40 to force liquid into the hydraulic portion of the system. Pumping continues until the hydraulic pressure builds up sufficiently to substantially equal the gas pressure, in which event the pressure in both bourdon tubes is essentially the same, causing the arm 55 to shift to a position opening the switch and the solenoid circuit, allowing the spring 44 to shift the valve 43 to its closed position, thereby discontinuing the supply of liquid to the hydraulic portion of the system.

In the event that the pressure of the liquid 15 in the system increases to a predetermined value above the pressure of the gas 16 in the accumulators 17, the excess liquid pressure is reduced. As illustrated, a differential valve 80, which can be of any suitable type, may have a body 81 provided with a liquid inlet port 82 connected to the liquid line 58, a liquid outlet port 83 being connnected to a line 84 leading to the reservoir 380. A piston 85 is shiftable in the body to control flow of liquid between the ports 82, 83, this piston being urged toward a closed position by a spring 86 and also by the pressure of the gaseous medium 16 acting thereon, which communicates with one end of the body through the line 36a. The line 58 has a branch 87 leading to the other end of the body 81, so that the liquid pressure acts on the other end of the piston 85, urging it toward an open position against the combined force of the spring 86 and gas pressure, permitting liquid flow between the inlet and outlet ports 82, 83.

When the pressure of the liquid medium 15 increases to a value exceeding the combination of the pressure of the gaseous medium 16 and the pressure equivalent of the force of the spring 86, the piston 85 shifts to a position opening the differential valve (upwardly in FIG.

3), allowing liquid to flow from the line 58 through the discharge line 84 to the reservoir 38a, thereby decreasing the liquid pressure. As an example, such excess liquid pressure differential could be developed during operation of the system by the liquid shifting the accumulator pistons 24 upwardly in the cylinders 22 to their upper ends, the liquid pressure then substantially exceeding the pressure of the gas 16. Again, in initially filling the system with liquid 15, the pistons 24 could be shifted upwardly in the cylinders 22 to their maximum extents, the liquid pressure continuing to increase and effecting opening of the differential valve 80 to bleed the excess liquid back to the reservoir 38a.

If the liquid pressure were to increase above a safe maximum preset value, substantially above the normal operating pressure of the liquid, the excess pressure will bleed through the line 58 and through a pressure relief valve 60, set at the desired value, into the reservoir 38a, thereby effecting a withdrawal of liquid from the hydraulic portion of the system.

Protective gas, such as nitrogen, under the desired pressure is disposed in the compensator cylinders 61 above the pistons 14. Thus, such gas may be stored in a vessel 62, flowing therefrom through a manually operated valve 63 and through a pressure regulator 64 into a line 65 that communicates with the ullage space 66 in the reservoir 38a and to a line 67 running to the head ends 61 of the compensator cylinders, the pressure of such gas being observable on a suitable pressure gauge 68 connected to the line or tube 67. The gas under pressure acts as a protective device to the compensating unit D, to afford a gaseous medium and cushion in the cylinders in the event the pistons 14 move upwardly to the maximum extent of their stroke. Moreover, as the pistons 14 move relatively in the cylinders, the gas volume in the cylinder spaces 61 thereabove will change, the gas pressure changing accordingly and in a linear manner. By observing this gas pressure on the gauge 68, the operator will receive an indication of the positions of the pistons 14 in the cylinders 13.

A valve 70 may be connected to the line 30 to bleed off the pressure in the manifold when the shut-off valves 31 leading from the gas storage tanks 29 are closed, the manifold pressure being observable on a gauge 71. The valve 63 in the line leading from the storage bottle 62 may be closed whenever the storage bottle is to be changed for replenishment of the gas supply therein.

in the operation of the apparatus illustrated in FIGS. 1 to 3, the accumulators 17 are charged with gas 16 at the desired pressure, there being sufficient liquid 15 in 'the system such that the same pressure is imposed on the liquid in the compensating cylinders 13, which exerts an upward force on the pistons 14 and on the drill string S, the supported load being carried from the cylinders 13 to the traveling block E, and from the lines H to the crown block J. Drilling proceeds through appropriate rotation of the rotary table T, with drilling mud being pumped down the drilling string, returning in a known manner to the floating vessel V through a marine conductor pipe (not shown in the interest of simplicity of illustration) extending from the drilling vessel V to the well bore W. In the event the floating vessel were to shift vertically, for example, rise, the

cylinders 13 will move upwardly along the piston rods 14 and pistons 12, the liquid therein remaining at substantially the same pressure merely being forced through the lines 18, 20 into the lower portions 21 of the accumulators l7, forcing the floating pistons 24 upwardly to further compress the gas 16 to a small extent, which will effect some increase in the pressure of the liquid 15, but, as a practical matter, to too small an extent as to have any significant effect on the drilling weight imposed on the drill bit B. Similarly, should the vessel V move downwardly, the cylinders 13 would move downwardly therewith, the cylinder volume below the pistons 14 increasing, the gas 16 under pressure forcing the liquid 15 from the accumulators back into the compensator cylinders 13, while maintaining the required pressure on the liquid 15. Thus, within the operative stroke of the compensator apparatus, the drilling vessel can heave upwardly and downwardly relative to the drill string S without materially altering the drilling weight on the drill bit. The number of compensators 17 and their combined cross-sectional areas are preferably many times the annular areas of the pistons 14, so that a large liquid volume change in the cylinders 13 produces a much smaller volume change in each accumulator or expansion space 21, thereby effecting only relatively small movement of each piston 24 and change in the pressure of the gas in the upper portion 23 of the accumulator.

If the operator wishes to maintain the pistons 14 in an intermediate position within the cylinders 13, as, for example, midway along the length of their travel in the cylinders, he can observe the pressure on the gauge 68, which indicates the position of the piston in the cylinders, and, as drilling progresses, feed off line H from the drawworks in the usual manner of drilling on land, so that the travelling block E and compensator D are lowered to the same extent as the footage drilled. However, the operator may, if he desires, merely retain the drawworks in a set position, the pistons 14 merely lowering along the cylinders 13 as the drilling proceeds, while the pressure of the liquid 15 is maintained for the purpose of supporting the required portion of the total drilling weight of the drill string S, insuring that the remainder of such drilling weight will be imposed on the drill bit B. When the pistons 14 descend sufficiently, the driller can then feed line H off the draw works, allowing the cylinders 13 to shift downwardly along the pistons 14 and piston rods 12 once again. However, during such downward shifting, the gas 16 under pressure in the accumulators 17 will feed liquid 15 back into the compensator cylinders 13, while maintaining such liquid within the desired pressure range, to hold the drilling weight on the drill bit substantially at a constant value.

Thus, it is apparent that the gas under pressure will maintain the required liquid pressure in the cylinders 13, and that the cylinders 13 and pistons 14 can shift vertically relative to each other to compensate for the vertical movement of the floating vessel V under wind, tide and wave action. As an example, the cylinders 13 and piston rods 12 may be of such length as to permit a safe vertical movement of the floating vessel of about 10 feet, with the drilling string S being maintained in tension to the desired value by the liquid 15 under pressure.

The drilling weight on the bit can be varied at any time, merely by changing the pressure of the gas 16 in the accumulators 17. This will, of course, result in the same pressure being imposed on the liquid 15, inasmuch as the gas pressure is transmitted through the floating pistons 24 to the liquid.

During the operation of the apparatus, the gas and liquid pressures are normally the same. However, should the system become unbalanced, as, for example, by the gas pressure driving the floating pistons 24 downwardly against the bottom ends of the accumulator cylinders 22, the liquid pressure could then drop below the gas pressure, resulting in contraction of the hydraulic bourdon tube 57, effecting closing of the differential pressure switch 48, as described hereinabove, completing the circuit to the solenoid 49 and effecting opening of the solenoid valve 43, whereupon the stall type of air motor 45 is permitted to operate and cause the pump 40 to deliver additional liquid under pressure into the system, until the liquid pressure is brought back again to a condition of equality with the gas pressure, the pistons 24, of course, being shifted upwardly into an intermediate position within the accumulator cylinders.

If, however, the accumulator pistons 24 are forced upwardly by the compensator pistons 14 into engagement with the tops of the accumulator cylinders 22, the liquid pressure could increase above the setting of the pressure relief valve 60, which will then open to allow the excess liquid to discharge into the reservoir 38a, the gas 16 under pressure then being capable of returning the floating pistons 24 to their intermediate positions.

Both the gas and liquid pressures can increase at the same time, such increase in pressure resulting in expansion of the bourdon tubes 56, 57. However, expansion of both tubes, although capable of shifting the switch arm 55 to some extent toward the contacts 52, 53, will not shift the switch arm sufficiently to complete the circuit to the solenoid 49 of the valve 43.

Assuming that the gas pressure were to increase above the hydraulic pressure (rather than the hydraulic pressure decreasing as described above), then the gas bourdon tube 56 would expand and close the switch 48, completing the circuit to the solenoid 49 to open the valve 43, whereupon the pump 40 can deliver additional liquid into the system, increasing the liquid pressure and restoring the balance between the gas and liquid pressures, the switch opening and the spring 44 shifting the valve 43 back to it closed condition.

It is, thus, apparent that an apparatus has been provided that compensates for the vertical movement of the vessel V due to ocean or sea conditions while drilling a well bore. The same action would occur while performing other operations in the well bore, such as running pipe, fishing or logging, or, for that matter, during the connection of subsea well equipment at the subaqueous floor F. The compensating apparatus will maintain the desired tension on the equipment, despite the heaving of the vessel. Such desired tension can be varied by appropriately adjusting the regulator valve 34, and, therefore, the pressure of the gas 16.

The same system has other specific applications, such as a motion compensating device as applied to a crane. As illustrated in FIG. 4, the boom 100 of a crane is pivoted to and extends from a supporting structure 101 which may swivel on a base 102. A load carrying hook 103 depends from a travelling block 104, there being a line 105 passing over suitable sheaves 106 in the block and at the end of the boom, and over a pulley 107 on the frame of the crane, this line passing over another pulley 108 rotatably carried by a piston rod 109 secured to a piston 110 vertically reciprocable in a cylinder 11] affixed to the frame 101 of the crane, the line passing from the rod pulley 108 to a drawworks 112. Another drawworks 113 can operate through a suitable cable and pulley system 114 to raise or lower the crane boom about its pivot axis.

The same compensating system as illustrated in FIG. 3 can be used in connection with the liquid 15 in the cylinder 111 that provides an appropriate upward pull, and, therefore, tension, on the cable or line running to the travelling block 104. The pressure of the liquid 15 in the cylinder is maintained substantially constant, generally corresponding to the load suspended from the hookl03. If, for example, the load is being lowered onto a barge floating in a heavy sea, the heaving of the barge might cause the load to strike its deck. However, such impacting action is minimized considerably, inasmuch as any relieving of tension in the line 105 will result in the rapid elevation of the piston and rod 109 in the cylinder 111 and the reestablishment of the tension in the line 105, thereby minimizing the opportunity for damage to the load.

We claim:

1. In apparatus for maintaining a predetermined stress in a running string: cylinder means; piston means slidable in said cylinder means; one of said means being adapted for operative connection to the running string; the other of said means being adapted for operative connection to a support; means for maintaining a liquid medium under pressure in said cylinder means on one side of said piston means as said piston means and cylinder means move longitudinally relative to one another in both longitudinal directions; said pressure maintaining means comprising a gaseous medium under pressure exerting its pressure force on said liquid medium, said pressure maintaining means further including an accumulator cylinder containing part of said liquid medium in a first portion thereof and said gaseous medium in a second portion thereof, and a piston movable in said cylinder between said mediums and engaged thereby to transmit pressure therebetween, means for feeding said gaseous medium under pressure into said second portion, means for conducting said liquid medium between an end of said first portion opposite said piston and said cylinder means; a pump for increasing the pressure and quantity of .said liquid medium in said first portion and cylinder means; a valve for controlling the flow of liquid medium from said pump to said first portion and cylinder means; and means responsive to a substantially higher pressure of said gaseous medium in said second portion resulting from engagement of said piston with said end of said first portion for opening said valve to permit said pump to feed additional liquid medium into said first portion and cylinder means.

2. In apparatus for maintaining a predetermined stress in a running string disposed in a well bore and which is supported by a rig including a suspension mechanism: elongate cylinder means; piston means slidable in said cylinder means; one of said means having a device thereon for operative connection to the running string; the other of said means having a device thereon for operative connection to the suspension mechanism, whereby said cylinder means and piston means are disposed between the suspension mechanism and running string; means for maintaining a liquid medium under pressure in said cylinder means on one side of said piston means as said piston means and cylinder means move longitudinally relative to one another in both longitudinal directions, whereby the stress of the running string and of said one means is transmitted through the liquid to said other means and suspension mechanism; said pressure maintaining means comprising a gaseous medium under pressure exerting its pressure force on said liquid medium, said pressure maintaining means further including an accumulator cylinder containing part of said liquid medium in a first portion thereof and said gaseous medium in a second portion thereof, and piston movable in said cylinder between said mediums and engaged thereby to transmit pressure therebetween, means for feeding said gaseous medium under pressure into said second portion, means for conducting said liquid medium between an end of said first portion opposite said piston and said cylinder means; a pump for increasing the pressure and quantity of said liquid medium in said first portion and cylinder means; a valve for controlling the flow of liquid medium from said pump to said first portion and cylinder means; and means responsive to a substantially higher pressure of said gaseous medium in said second portion resulting from engagement of said piston with said end of said first portion for opening said valve to permit said pump to feed additional liquid medium into said first portion and cylinder means.

3. In a crane: a cable and pulley system for supporting a load; means for moving the cable of said system to raise and lower the load; and means for maintaining a predetermined stress in said cable comprising cylinder means, piston means slidable in said cylinder means, one of said means being adapted for operative connection to the cable, the other of said means being adapted for operative connection to a support, means for maintaining a liquid medium under pressure in said cylinder means on one side of said piston means as said piston means and cylinder means move longitudinally relative to one another in both longitudinal directions, said pressure maintaining means comprising a gaseous medium under pressure exerting its pressure force on said liquid medium, said pressure maintaining means further including an accumulator cylinder containing part of said liquid medium in a first portion thereof and said gaseous medium in a second portion thereof, and a piston movable in said cylinder between said mediums and engaged thereby to transmit pressure therebetween, means for feeding said gaseous medium under pressure into said second portion, means for conducting said liquid medium between an end of said first portion opposite said piston and said cylinder means; a pump for increasing the pressure and quantity of said liquid medium in said first portion and cylinder means; a valve for controlling the flow of liquid medium from said pump to said first portion and cylinder means; and

means responsive to a subs tantially higher pressure of said gaseous medium in said second portion resulting from engagement of said piston with said end of said first portion for opening said valve to permit said pump to feed additional liquid medium into said first portion and cylinder means.

4. In apparatus as defined in claim 1; a second gaseous medium under superatmospheric pressure in said cylinder means on the other side of said piston means; and indicating means responsive to and operable by the pressure of said second gaseous medium for indicating the position of said piston means in said cylinder means. 

1. In apparatus for maintaining a predetermined stress in a running string: cylinder means; piston means slidable in said cylinder means; one of said means being adapted for operative connection to the running string; the other of said means being adapted for operative connection to a support; means for maintaining a liquid medium under pressure in said cylinder means on one side of said piston means as said piston means and cylinder means move longitudinally relative to one another in both longitudinal directions; said pressure maintaining means comprising a gaseous medium under pressure exerting its pressure force on said liquid medium, said pressure maintaining means further including an accumulator cylinder containing part of said liquid medium in a first portion thereof and said gaseous medium in a second portion thereof, and a piston movable in said cylinder between said mediums and engaged thereby to transmit pressure therebetween, means for feeding said gaseous medium under pressure into said second portion, means for conducting said liquid medium between an end of said first portion opposite said piston and said cylinder means; a pump for increasing the pressure and quantity of said liquid medium in said first portion and cylinder means; a valve for controlling the flow of liquid medium from said pump to said first portion and cylinder means; and means responsive to a substantially higher pressure of said gaseous medium in said second portion resulting from engagement of said piston with said end of said first portion for opening said valve to permit said pump to feed additional liquid medium into said first portion and cylinder means.
 2. In apparatus for maintaining a predetermined stress in a running string disposed in a well bore and which is supported by a rig including a suspension mechanism: elongate cylinder means; piston means slidable in said cylinder means; one of said means haVing a device thereon for operative connection to the running string; the other of said means having a device thereon for operative connection to the suspension mechanism, whereby said cylinder means and piston means are disposed between the suspension mechanism and running string; means for maintaining a liquid medium under pressure in said cylinder means on one side of said piston means as said piston means and cylinder means move longitudinally relative to one another in both longitudinal directions, whereby the stress of the running string and of said one means is transmitted through the liquid to said other means and suspension mechanism; said pressure maintaining means comprising a gaseous medium under pressure exerting its pressure force on said liquid medium, said pressure maintaining means further including an accumulator cylinder containing part of said liquid medium in a first portion thereof and said gaseous medium in a second portion thereof, and piston movable in said cylinder between said mediums and engaged thereby to transmit pressure therebetween, means for feeding said gaseous medium under pressure into said second portion, means for conducting said liquid medium between an end of said first portion opposite said piston and said cylinder means; a pump for increasing the pressure and quantity of said liquid medium in said first portion and cylinder means; a valve for controlling the flow of liquid medium from said pump to said first portion and cylinder means; and means responsive to a substantially higher pressure of said gaseous medium in said second portion resulting from engagement of said piston with said end of said first portion for opening said valve to permit said pump to feed additional liquid medium into said first portion and cylinder means.
 3. In a crane: a cable and pulley system for supporting a load; means for moving the cable of said system to raise and lower the load; and means for maintaining a predetermined stress in said cable comprising cylinder means, piston means slidable in said cylinder means, one of said means being adapted for operative connection to the cable, the other of said means being adapted for operative connection to a support, means for maintaining a liquid medium under pressure in said cylinder means on one side of said piston means as said piston means and cylinder means move longitudinally relative to one another in both longitudinal directions, said pressure maintaining means comprising a gaseous medium under pressure exerting its pressure force on said liquid medium, said pressure maintaining means further including an accumulator cylinder containing part of said liquid medium in a first portion thereof and said gaseous medium in a second portion thereof, and a piston movable in said cylinder between said mediums and engaged thereby to transmit pressure therebetween, means for feeding said gaseous medium under pressure into said second portion, means for conducting said liquid medium between an end of said first portion opposite said piston and said cylinder means; a pump for increasing the pressure and quantity of said liquid medium in said first portion and cylinder means; a valve for controlling the flow of liquid medium from said pump to said first portion and cylinder means; and means responsive to a substantially higher pressure of said gaseous medium in said second portion resulting from engagement of said piston with said end of said first portion for opening said valve to permit said pump to feed additional liquid medium into said first portion and cylinder means.
 4. In apparatus as defined in claim 1; a second gaseous medium under superatmospheric pressure in said cylinder means on the other side of said piston means; and indicating means responsive to and operable by the pressure of said second gaseous medium for indicating the position of said piston means in said cylinder means. 